Gold plating

ABSTRACT

Process for electrodeposition of gold from aqueous gold-plating baths which are substantially free from alkali metal ions and which are comprised of ammonium gold cyanide as the source of gold.

Inventors Appl. No.

Filed Patented Assignee United States Patent Carl D. Keith Summit; 1 Alfred J. Haley, Jr., Florlgam Park; Kenneth D. Baker, Bridgewater, Somerville, all of NJ.

July 3, 1969 Nov. 2, 1971 Engelhard Minerals 81 Chemical Corporation Newark, NJ.

sou) PLATING 6 Claims, 2 Drawing Figs.

US. Cl

Primary Examiner-G. L. Kaplan Attorneys-Samuel Kahn and Miriam W. Leff ABSTRACT: Process for electrodeposition of gold from aqueous gold-plating baths which are substantially free from alkali metal ions and which are comprised of ammonium gold cyanide as the source of gold PATENTED NUV2 l97l BATH I E E E. ..E BATH 2 o- BATH 3 M BATH 4 A .7 N v m. Y A.E E.E CATHODE R 2 4 EFFIC'ENCY -l W.

cw 8O 2 3 V .3 MUM 60 .E ,'".M E. 3 E A I 3 5 7 9 CURRENT DENSITY (AsFI FIG. 1

BATH I 0 E. h BATH 2 0 BATH 3 A E E 3- BATH 4+ IOO CATHODE g l I EFFICIENCY 4 -i 60 #i 4 \A 3 FIG. 2

CURRENT DENSITY (A S F INVENTORS CARL D. KEITH ALFRED J. HALEY,Jr. KENNETH D. BAKER A QR GOLD PLATING BACKGROUND OF THE INVENTION This invention relates to the electrodeposition of gold from a novel bath which has long life, is relatively economical to operate, and produces smooth lustrous gold deposits. It is well known to electroplate gold from baths using gold cyanide complex salts as the source of the gold. Gold cyanide baths have been operated under alkaline, neutral, and acid conditions, but the present trend is to operate in the neutral or acid range, as illustrated in U.S. Pat. No. 2,812,299 and 3,423,295. It is standard practice in the art to control pH, improve the conductivity of the bath, and improve the deposit by using additives in the bath. Examples of additives are water-soluble organic and inorganic compounds, phosphates, sulfamates, tartrates, citrates, formates, sulfates, and the like.

In the past it has been customary to operate gold-plating baths using either potassium or sodium gold complex salts as the source of gold, and potassium gold cyanide has been preferred because of its comparatively high solubility in water. Although it has been taught that ammonium gold cyanide may be used, ithas been the practice to use only either the potassium or sodium salt because of their easy preparation and their availability. During the operation of conventional processes, as gold deposits from the plating bath it is replenished by adding the alkali metal gold complex salt to restore the gold concentration in solution. As the plating proceeds alkali metal salts, e.g. potassium cyanide, are formed and there is a tendency for the pH to rise, whereupon the pH is usually adjusted by the addition of an acid such as phosphoric, tartaric, citric, sulfuric, sulfamic, and the like. After a bath has been replenished several times, it has poor current efficiency and produces gold plate of poor quality and it is necessary to discard the solution. Indeed this is a major problem in depositing heavy coatings, e.g. platings having a thickness of 5 to 50 mils, since the baths are run continuously for relatively long periods of time and preferably operated at high current densities. One of the main causes of the deficiencies in the baths is the buildup of certain cations, i.e. alkali metal ions, and anions ordinarily added during operation of the plating bath. This buildup is evidenced by a change'in the specific gravity of the solution. Despite the deficiencies in the prior art baths, heretofore it was not recognized that it would be advantageous to use a bath that is substantially free from alkali metal ions.

It has now been found that ammonium gold cyanide plating baths substantially free of alkali metal ions are particularly effective for plating gold from baths operating at a pH in the range of about 5 to 8 and in the temperature range of about to 90 C. Such baths operate with substantially constant and high cathode efficiency over a wide range of current densities, thereby facilitating the formation of relatively even uniform plate on certain complex structures. It has further been found that the specific gravity of such baths remain substantially constant during the plating operation and the baths have long life. The present baths are effective for producing both thin and very thick coatings e.g. from a few microinches in thickness to 20 mils and thicker. The gold deposits obtained with these baths compare favorably with those from conventional plating baths.

INVENTION According to the present invention gold is electroplated from a bath comprising an aqueous solution containing as the source of gold ammonium gold cyanide and said solution is substantially free of alkali metal ions. The ammonium gold cyanide is used in the makeup bath and as the replenisher.

In addition to the ammonium gold cyanide the plating bath contains at least one other additive to improve the conductivity of the bath and maintain the pH in the range of about 5 to 8 during the plating operation. Among the compounds that may be used are soluble phosphates, sulfamates, citrates, tartrates, sulfates, etc., and mixtures thereof. Such additives are well known in the art. The provision in the bath of the present in vention is that such compounds are used in a form which does not produce a substantial amount of alkali metal ion in the bath. They may be used for example as ammonium or ammonium acid salts or acids. A bath containing ammonium sulfamate and ammonium dihydrogen phosphate and phosphoric acid to improve the conductivity and provide a pH in the range of about 5.5 to 6.5 has been found particularly satisfactory.

Conventional gold-plating baths often also contain brighteners and/or compounds of heavy metals other than gold, e.g., Ag, Zn, Cd, Sn, Pb, As, Sb, I-lg, Co, Ni, and other precious metals, which may or may not alloy with the gold plate. Such additives may also be present in the present plating baths so long as they do not produce a substantial amount of free alkali metal ions in solution.

The concentrations of the ammonium gold cyanide in the bath is not a critical feature of the plating baths of this invention. It may vary, for example, from about I gram per liter to near saturation, the choice being dictated by such factors as the desired thickness of the gold deposit, the current density used, and economic considerations. Very satisfactory coatings are obtained with baths containing 5 to 25g./l. of Au. It is well within the skill of the art to select a bath of suitable gold content.

1 Similarly it is considered within the skill of the plating art to choose a suitable concentration of additives. For example, the buffering and/or conductivity salts are used in a concentration of about 20 to I00 g./l. Generally the buffering salts are used in such concentration as to operate in the desired pH, which in the process of this invention is 5 to 8. Additionalammonium salts, ammonium acid salts, or acids may be added to increase the conductivity of a bath. The conductivity of the baths may be as low as about 0.025 mho/cm., measured at room temperature. However, particularly suitable baths, having a concentration of 8 to 12 grams of gold per liter have a conductivity in the range of about 0.040 to 0.055 mho/cm.

As noted above the addition of brighteners and other compounds to improve the coating or alloy with the gold is optional and well within the skill of the art. It will be understood that the ammonium salts and other alkali metal-free additives used in the plating baths of this invention may contain small quantities of alkali metals as impurities, especially when such compounds are produced on a commercial scale and the baths of the present invention may include such minor quantities of impurities.

In carrying out the process of this invention any of the conventional procedures now employed in the electroplating art, such as barrel plating, rack plating, strip plating, and electroforming may be used.

It is one of the advantages of this invention that the baths may be operated over a wide range of current densities with high cathode efficiency and the efficiency is substantially constant. The range of current densities for constant cathode efficiency is dependent upon the concentration of the solution used. Preferably a bath containing 12 grams/liter of gold would be operated at a current density of l to l0 a.s.f. A bath having a gold concentration of about 24 g./l. operates at a substantially constant cathode efficiency over a range of about 3 to 50 a.s.f.

The baths are suitably operated over a temperature range of about 20-90 C. The preferred range is about 30-75 C. The present baths are effective for producing thick as well as thin gold plate and they can be used in electroforming processes wherein the bath is operated continuously to produce heavy gold plates having a thickness of, for example, 5 to 50 mils.

PREFERRED PLATING BATHS Particularly satisfactory baths are aqueous solutions having.

the following compositions:

AMMONIUM BATH TYPE A Ingredient Grams/Liter Gold as ammonium gold cyanide l to 50 ammonium sulfamate it) to 80 ammonium dihydrogen phosphate It! to 80 acid to pH of 5 to 8 AMMONIUM BATH TYPE B Ingredient Grams/Liter Gold as ammonium gold cyanide l to 50 ammonium citrate 20 to I acid to pH of -8 As noted previously, brighteners, alloying agents, leveling agents, and thelike may be added to the bath.

PREPARATION OF AMMONIUM GOLD CYANlDE l. Using Gold Cyanide To an aqueous slurry of gold cyanide (containing 88% Au) is added a slight excess over the stoichiometric quantity of Nl-LOH to form the ammonium gold cyanide complex compound. A mixture of about 50:50 air and HCN is bubbled into the slurry and the gold cyanide dissolved. This solution is gently heated to expel excess HCN and an aqueous solution of ammonium gold cyanide is obtained.

ll. Using a Double Metal Complex of Gold and a Cation Exchange Resin As described inv copending application, Ser. No. 838,946, filed on July 3, 1969, ammonium gold cyanide may be formed by passing an alkali metal gold cyanide into contact with an ammoniated ion exchange resin. in this method a cation exchange resin in hydrogen form is first converted to the ammonium form. The cycle used in an embodiment of the disclosed preparation is as follows:

In a preferred method of preparation an aqueous solution of an ammonium salt, approximately 10 percent by weight ammonium sulfamate,-is passed through an ion exchange column containing a highly sulfonated resin in hydrogen form, Amberlite lR-l 20 (sold by Rohm and Haas Company) to convert the resin to the ammonium form. Thereafter, an aqueous solution of percent potassium gold cyanide is passed into the column, producing an effluent containing ammonium gold cyanide which is withdrawn from the column.

The ammonium gold cyanide prepared in this manner is virtually free of alkali metal ions. The resin can be regenerated to the hydrogen form and. the cycle repeated. The eluted acidified sulfamate can be regenerated to the ammonium salt and reused.

The following examples give typical plating baths and illustrate the advantages of the present invention.

EXAMPLE l Bath l Ammonium gold cyanide Ammonium sulhmsts l6.3 g. 37.5 g.

Diamrnonium acid phosphate 43.5 g. Water to make I liter of solution pH adjusted with phosphoric acid to 6.l

Bath 2 Potassium gold cyanide l7.6 g. Ammonium sulfamate 37.5 g. Diammonium scid phosphate 43.5 g. Water to make 1 liter of solution pH adjusted with phosphoric acid to 6.]

Bath 3 Potassium gold cyanide 17.6 g. Ammonium citrate 37.5 Potassium dihydrogen phosphate 43.5 Water to make I liter of solution pH adjusted with phosphoric acid to 6.l

Bath 4 Potassium gold cyanide l7.S g. Ammonium citrate 26.5 Tetraammonium ethylene diamine tetracetic acid 23 g.

Water to make I liter of solution pH adjusted with phosphoric acid to 6.l

EXAMPLE 2 Each of the four baths of example l was used in a test cell for electrodeposition of gold in the usual manner in cells operatingat a temperature in the range of 6065 C. at current densities of l, 3, 5, 7 and 10 a.s.f. The cathode efficiency, of the four baths was determined at each of the current densities. The cathode efficiency refers to the actual amount of gold plated on the cathode determined gravimetrically compared with the theoretical amount that should be plated according to Faradays law. The results are tabulated in table I and plotted in FIG. 1.

It will be seen from table I and FIG. 1 that the variation of cathode efficiency at different current densities between l a.s.f. and 10 a.s.f. is the least in bath 1; the maximum variation in each bath being 3 percent for bath l, 14 percent for bath 2, 14 percent for bath 3 and 25 percent for bath 4. The advantage of operating at a substantially unifonn cathode efficiency over the wide range of current densities is readily apparent to one skilled in the art. For example it is easier to deposit uniform coatings on substrates of certain complex shapes.

it will be noted that the uniform efficiency over the range of l to 10 a.s.f. was recorded for bath 1, having a gold content of 12 g./l. Bath 5, having a gold content of 30 g./l. and the following composition:

Ammonium gold cyanide 40.8 g. Ammonium sulfamate g. Diarnmonium Hydrogen Phosphate 87 g.

Water to make I liter of solution pH=7.25 m 7.3

and operated at 6675 C. had a cathode efficiency of 90 to 88.5 percent operating at current densities of 20 to 40 a.s.f. and said high constant cathode efficiency can be maintained over a range of about 5 to 50 a.s.f.

EXAMPLE 3 Bath 1 and bath 3 of example 1 were utilized to plate 120 grams of gold in cells operating at 60 C. and a current density of 5 a.s.f. Each of the baths was replenished with gold as Au wasdeposited to maintain the bath at 12 grams of Au per liter. Bath 1 was replenished with ammonium gold cyanide and bath 3 was replenished with potassium gold cyanide. in bath 3, in addition to replenishing with gold the pH had to be adjusted by addition of phosphoric acid to maintain it in the range of 5.2 to 6.5. Each of the baths had an initial density of 9.5 Be; The density of each of the baths was determined at various intervals during the test.

It was found that the density of bath 1, a bath in accordance with this invention remained constant at 9.5 Be over the life of the test. In bath 3, a conventional plating bath, the density continually increased and after 60 grams of gold were plated from solution the density had increased to 12 Be.

EXAMPLE 4 TABLE ll Cathode Efficiency of Baths Current Density After I g. Au/l. Deposited l a.s.l'. 94% 83% 92% 67% 3 asf. 90% 84% 77% 60% S a.s.l'. 90% 70% 60% 59% 7 a.s.l'. 94% 67% 46% 59% Table ll and FIG. 2 show that the cathode efficiency of bath 1, the bath of this invention, remained high and substantially uniform over the range of current densities tested. Baths, 2, 3, and 4 showed a marked decrease in cathode efficiency, particularly at higher current densities.

EXAMPLE 5 Bath l was used to deposit 0.0025 inches of gold on brass. The deposit was pale yellow, smooth, and had a hardness of Knoop, as determined using a microhardness tester. Surface roughness was determined to be less than 40 microinches.

The gold coated brass was heated to 450 C. for l hour without any discoloration of the deposit.

EXAMPLE 6 Bath 1 was used to form on brass, deposits of 25, 50, 75, and 100 microinches. The samples were kept in a hydrogen sulfide atmosphere for 1 week. No discoloration, pits, or cracks were observed.

lt will be appreciated that while the present invention has been described with reference to specific embodiments, modifications and substitutions can be made without departing from the spirit of this disclosure.

What is claimed:

1. A process for the electrodeposition of gold which comprises cathodically depositing gold at a temperature of about 20 to C. from an aqueous bath having a pH in the range of 5 to 8, said bath being substantially alkali metal ion free and containing as the source of gold ammonium gold cyanide in the amount of about 1 gram per liter to near saturation concentration, and said bath containing at least one additive compound, which is substantially alkali metal ion free, in an amount to provide the bath with a conductivity of at least 0.025' mho/cm. and to maintain the bath at said pH of 5 to 8 during the electrodepositing operation.

2. The process of claim 1 wherein the bath is operated continuously to produce a plating of a thickness of 5 to 50 mils.

3. A process of claim 1 wherein the ammonium gold cyanide is present in a concentration to give about I to 50 grams of gold per liter of solution.

4. An aqueous electroplating bath having a pH in the range of 5 to 8 for depositing gold, said bath being substantially free of alkali metal ions and containing about 1 gram per liter to near saturation concentration of ammonium gold cyanide as the source of gold.

5. An aqueous electroplating bath of claim 4 consisting essentially of l to 50 grams per liter of gold as ammonium gold cyanide, about 10 to 80 grams per liter of ammonium sulfamate, about 10 to 80 grams per liter of ammonium acid phosphates, and an acid to pH OF 5 TO 8.

6. An aqueous electroplating bath of claim 4 consisting essentially of about 1 to 50 grams of gold as ammonium gold cyanide, about 20 to grams per liter of ammonium citrate, and an acid to pH of 5 to 8. 

2. The process of claim 1 wherein the bath is operated continuously to produce a plating of a thickness of 5 to 50 mils.
 3. A process of claim 1 wherein the ammonium gold cyanide is present in a concentration to give about 1 to 50 grams of gold per liter of solution.
 4. An aqueous electroplating bath havinG a pH in the range of 5 to 8 for depositing gold, said bath being substantially free of alkali metal ions and containing about 1 gram per liter to near saturation concentration of ammonium gold cyanide as the source of gold.
 5. An aqueous electroplating bath of claim 4 consisting essentially of 1 to 50 grams per liter of gold as ammonium gold cyanide, about 10 to 80 grams per liter of ammonium sulfamate, about 10 to 80 grams per liter of ammonium acid phosphates, and an acid to pH OF 5 TO
 8. 6. An aqueous electroplating bath of claim 4 consisting essentially of about 1 to 50 grams of gold as ammonium gold cyanide, about 20 to 100 grams per liter of ammonium citrate, and an acid to pH of 5 to
 8. 